Optical coherence tomography (OCT) is an optical signal acquisition and processing method. It captures micrometer-resolution, three-dimensional images from within optical scattering media (e.g., biological tissue). Optical coherence tomography is an interferometric technique, typically employing near-infrared light. The use of relatively long wavelength light allows it to penetrate into the scattering medium. Confocal microscopy, another similar technique, typically penetrates less deeply into the sample. An OCT system is essentially a broadband interferometer that is often used for depth section inspection of tissues in biological and medical applications.
Depending on the properties of the light source (superluminescent diodes and ultrashort pulsed lasers have been employed), Optical coherence tomography has achieved sub-micrometer resolution (with very wide-spectrum sources emitting over a ˜100 nm wavelength range).
Optical coherence tomography is one of a class of optical tomographic techniques. A relatively recent implementation of optical coherence tomography, frequency-domain optical coherence tomography, provides advantages in signal-to-noise ratio, permitting faster signal acquisition. Optical Coherenced Tomography (OCT) is often used as an optical method in biology and medicine. Commercially available optical coherence tomography systems are employed in diverse applications, including art conservation and diagnostic medicine, notably in ophthalmology where it can be used to obtain detailed images from within the retina.
In inspection in general and optical metrology in particular there is an ever-increasing importance to the accurate determination of the sensing head distance from the inspected target. This is because of the frequent connection between system—target distance and overall measurement accuracy. As OCT operates essentially in the same way as a Linnik interferometer, OCT can be considered for use as a focus sensor like the Linnik interferometer.
FIG. 1 illustrates an example of a conventional metrology tool 100 that can be used to perform scatterometry measurements on the target structures at the surface of a sample 124. The metrology tool 100 may include a light source (not shown) optically coupled to illumination optics 114, potentially with angular/spatial dynamic control, for generating a probe beam 115 of radiation. The probe beam 115 may be turned towards the sample 124 with a 50/50 beam splitter 118. The probe beam 115 may be focused onto the surface of the sample 124 with a main objective 122. Probe beam radiation scattered from the target is collected and collimated by the objective 122 and at least a fraction of the sample beam passes through the beam splitter 118 and up an optical column of the tool 100. The fraction of the sample beam may be passed through relay lenses 104 and 106 and focused on an image detector 102, such as a charge-coupled device (CCD).
The metrology tool 100 includes an OCT focusing system having a second beam splitter 108, (sometimes called a focus beam splitter) a focus detector lens 110, a focus detector 112, a reference objective 120, and a reflector 121 that are arranged in a Linnik-type interferometer configuration. In a Linnik-type interferometer, the reference objective 120 has complementary optical properties to the main objective 122 so that the optical path length of the main and reference arms match.
A mechanical shutter 109 selectively opens and closes an optical path to the reference objective 120 between the beam splitter 118 and the reference objective 120, which focuses a reference beam onto the reflector 121. The reference beam passes back through the reference objective 120 toward the beam splitter 118.
Another fraction of the probe beam power (referred to as the sample beam) is reflected from the sample 124, and is directed toward the beam splitter 118 where it interferes with the reference beam. The resulting interference beam is directed to the focus beam splitter 108, which directs the interference beam towards the focus detector lens 110 which focuses the interference beam on a focus detector 112. Interference fringes are detected at the focus detector 112 as a result of interference between the sample beam and the reference beam. The interference fringes can be analyzed to detect whether the probe beam 115 is in focus at the sample 124.
However, OCT focus implemented as illustrated in FIG. 1 has some disadvantages. First, in a Linnik-type configuration the reference objective 120 has similar optical performance to the main objective 122 and is, therefore, expensive. The fringe quality depends on the optical matching between two complicated objectives (mainly limited by spherical aberrations). The mechanical shutter 109 is often a source of noise that can cause vibration. Because the interferometer is sensitive to temperature, the beam splitter must often be mounted with a piezoelectric tube (PZT) mount to compensate for temperature changes. This adds to the complexity and expense of the tool 100. Furthermore, because the focusing system uses the same illumination as is used for metrology, the spectrum available for focusing is constrained to the bandwidth used for metrology and does not enable the use of the widest possible spectral bandwidth for focusing. This is disadvantageous because a short coherence length is desired for the OCT-based focusing system and coherence length is inversely related to the bandwidth of the illumination used.
Furthermore, a Linnik interferometer configuration, such as that shown in FIG. 1, is sensitive to the system's environment, especially vibration which limits its precision. In addition, the need for a focus detector beam splitter has usually some effect on optical performance of the metrology tool that has to be monitored and calibrated away.
It is within this context that embodiments of the present invention arise.